Hi,
I'm facing this issue even if I follow instructions from the image attached bellow.
I correctly downloaded the file, uploaded it in /api and it's named .htaccess
But I still can not pass to management page of the REST API.

Visiting the /api page return
{ "errorCode": "2S290\/6", "errorMessage": "NO_API_KEY" }
Regards.

You would not be able to just visit /api without providing a key. If you are not getting past that page, you need to check why rewriting isnt working there. First of all I would check your main htaccess to see if there is anything additional there, as there are multiple redirects causing issues on your site in other places too

.htaccess (main)

<IfModule mod_rewrite.c>

Options -MultiViews

RewriteEngine On

RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f

RewriteRule \.(js|css|jpeg|jpg|gif|png|ico|map)(\?|$) /404error.php [L,NC]

RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule . /index.php [L]

</IfModule>

You would therefore need to check this with your hosting company

That .htacess file you posted is the default one for Friendly URLs (FURLs) that goes in the root directory (where conf_global.php is).

The one to enable the APIs goes inside the /api folder and is completely different. It should be something like this:

<IfModule mod_setenvif.c>
SetEnvIf Authorization .+ HTTP_AUTHORIZATION=$0
</IfModule>
<IfModule mod_rewrite.c>
Options -MultiViews
RewriteEngine On
RewriteBase /api/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* index.php [E=HTTP_AUTHORIZATION:%{HTTP:Authorization},L]
</IfModule>

Yes. That why I did not post it since it was not edited as well in the api folder.
Problem solved, request was blocked by cloudflare.